Semiconductor detectors are among the most widely used radiation detection devices. They have found applications ranging from space physics to medicine, to defense and homeland security. Semiconductor radiation detectors generally operate by absorbing a quantum of gamma-ray or a neutron and converting the radiation energy into a number of electron-hole pairs that is proportional to the absorbed energy. After the conversion, the motion of the electrons and holes induce electrical signals on detector electrodes. Semiconductor detectors have a number of advantages over other types of particle detectors. For instance, the energy needed to produce an electron-hole pair is only about one-tenth that needed to produce ionization in a gas (e.g., in a Geiger counter). Also, since semiconductors are much denser than a gas, these devices can be made much smaller than a Geiger counter or a cloud chamber.
For semiconductor gamma-ray detectors, the energy resolution is typically one to two orders of magnitude better than gas detectors and scintillators. Most commonly, gamma-ray detectors use germanium (Ge) and cadmium telluride (CdTe)/cadmium zinc telluride (CdZnTe) as the detector material. The interaction of gamma-rays in these semiconductor materials creates electron-hole pairs, the number of which depends on the energy of the incident radiation. The energy information can therefore be extracted by collecting the electron-hole pairs with the application of an external electric field. Semiconductor materials suitable for gamma-ray detection should have high atomic number (Z) for efficient radiation-atomic interactions, a large enough band-gap with high resistivity to maintain low noise associated with leakage current, high crystalline quality with low probability for recombination and trapping, and high carrier mobility for both electrons and holes.
The use of germanium (Ge) crystals cooled to liquid nitrogen temperatures appears to be at present the only choice for optimum performance with respect to efficiency and energy resolution. A major drawback, due to the small band gap of germanium, is that Ge-based gamma-ray detectors have to be operated at low temperature. On the other hand, the moderate need for cooling of CdZnTe (CZT) detectors stems from the fact that the band gap of CZT is significantly larger than that of Ge, which gives a smaller leakage current and hence can be operated at room temperature. Cd1-xZnxTe has a direct energy gap for all alloy compositions and is tunable from 1.5 to 2.3 eV at room temperature, for x=0 to 1 respectively. However, the problem with most high-Z semiconductors is the high probability of charge trapping which, depending on the position of the interaction between the electrodes, leads to incomplete charge collection.
For Ge, the density of trapping centers is extremely low and only a negligible fraction of free charges is trapped. The relatively small hole effective mass results in an uncommonly high hole mobility. As a result, the combination of low defect density and long carrier lifetime is responsible for the exceptionally good performance of Ge detectors (at low temperature). CZT on the other hand has a much higher density of trapping centers due to defects. Since the mobility of the holes in CZT is low, the transit time they need to reach the cathode is significant compared to the hole lifetime. Hence the number of free holes is seriously reduced as they drift across the detector. The effect of trapping is therefore a reduction in the amplitude of the output signal that is strongly dependent on the depth of the interaction site. In the output spectrum this gives rise to serious spectral distortion. Although there are a number of techniques developed to correct or compensate for this trapping effect, the performance of CZT detectors has not yet reached the level of which it is theoretically capable due to material quality and is still far from that of Ge detectors.
Solid-state semiconductor neutron detectors offer the advantages of low-power operation and compactness as compared to gas-filled or scintillation neutron detectors. However, neutrons only interact weakly with most known semiconductor detector materials. Neutron detectors function by converting the non-ionizing neutrons into more easily measured charged particles. This is typically accomplished by employing a high neutron cross-section converter material (e.g., 6Li, 10B, or 157Gd) that yields energetically charged particles upon capturing a neutron. Three very common neutron interactions that are used for a variety of neutron detectors are the 6Li(n, α)3H reaction, 10B(n, α)7Li reaction, and the 157Gd(n, γ)158Gd reaction. The energetically charged particles created then produce free carriers in the detector. The free carriers move in response to an applied electric field, producing a current that serves as the measure that a neutron interaction has occurred. However, unlike gamma-ray detection where semiconductor detectors based on Ge and CdZnTe are available, no comparable semiconductor neutron detector materials have been developed.
To fabricate detectors having good charge collecting efficiency and the ability to stop a large fraction of incident photons, high purity, homogeneous, defect-free material is required. For gamma-ray detectors, unlike silicon or gallium arsenide, it is very difficult to grow large size single crystals of CZT, which have been made traditionally from crystals that are grown by a high pressure method. In high pressure growth, crystals grow from a melt (at high temperature, greater than 1100° C.) of nearly equal quantities of cadmium and tellurium, with small cadmium excess, which generates a high vapor pressure. While the high pressure method yields quality detectors, the crystal uniformity is limited and the detector yield is low. Other than a few variations of the high pressure crystal growth method, there have been no reports of effective alternatives to grow high quality crystalline semiconductor materials for radiation detectors.
Incomplete charge collection, due to poor charge transport properties of the detector materials, degrades the energy resolution and lowers the effective photo peak efficiency for gamma-rays as well as for neutrons. Using CdTe as an example, the life time, (τ) and mobility (μ) of the electrons and holes that determine the detector's charge collection efficiency and affect detector's performance have been reported to be τe=3 μs, τh=2 μs, μe=1100 cm2/Vs, and μh=100 cm2/Vs, respectively. The relatively low hole mobility and life time cause hole trapping and consequent loss in signal. The amount of charge loss in a single particle detection event, and hence the signal strength, depends on the interaction depth of energy absorption. In energy sensitive measurements this gives rise to an asymmetric broadening of the spectral peaks toward the low energy direction.
Charge collection efficiency can in principle be improved by increasing the electric field strength in the detector crystal (according to the Hecht equation that describes induced charge signal at the anode electrode of standard planar radiation detectors). However, as long as the detector electrodes form ohmic contacts with the CdTe crystal, the detector leakage current is directly proportional to the electric field. The leakage current shot noise eventually becomes the dominant noise source and prohibits higher electric fields. Meanwhile, CdZnTe was developed to reach higher resistivity and lower leakage current than are possible with CdTe. Varying the Zn concentration, the band-gap can be stretched and the resistivity increased (to 1011 Ωcm). However, CdZnTe has the drawback of lower hole life time (τe=5 μs, τh=200 ns, μe=1350 cm2/Vs and μh=120 cm2/Vs). In this case, AlSb as well as GaAs have higher hole mobilities. However, its carrier lifetime is far from what needed to achieve good energy resolution.
There have been significant efforts to develop detector techniques based on the concept of single polarity charge-sensing in order to maintain good energy resolution. In the meantime, CdTe detectors with Schottky type rectifying metal contacts have been introduced. Reverse biasing a Schottky type CdTe detector reduces the leakage current and the detector can operate at higher electric fields. While these improvements have been able to increase the energy resolution to certain extent, there remains a clear need to develop a new paradigm in order to further improve the performance of radiation detectors by orders of magnitude.